The male koala has a large pink two-pronged penis. The female koala has three vaginas and two uteruses (see photos below). The mating season for koalas is between September and March, during which time male koalas make loud bellowing mating calls. These calls are unique to each male and advertise its size and fitness to female koalas nearby. At about this time, the female koala will wean away her prior year's offspring and prepare herself for the new mating season. Interestingly it is the female koala who chooses her partner and it is not the largest, most dominant male that she may choose. Instead, the female identifies her previous suitors by their mating calls and avoids those she has previously bred with, as she prefers a new partner each mating season.
Koala sex occurs at night. Copulation is a rather noisy affair, with much bellowing and grunting. The much larger male mounts the female from behind, inserts his two-pronged penis into her cloaca, and impregnates her.
Male koalas have a long two-pronged penis (bifurcated penis). The end of this double-barrelled penis splits into two separate shafts (see photo). During copulation, each shaft enters the female's corresponding left and right vaginas to impregnate her. When not in use, the male koala's penis is safely tucked away inside its cloaca.
Koalas have a fur-covered non-pendulous scrotum–meaning their scrotum (balls) is held close to their bodies and doesn't dangle. Also, the male koala's scrotum is in front of his penis. (Most animals have the penis in front and the scrotum behind it).
The female koala has three vaginas and two uteruses (uteri). The two outermost vaginas are used for sperm transportation to the two uteruses above them. Babies are born through the middle vagina. By contrast, female placental mammals have only one uterus and one vagina. (See photo).
The male koala's sperm contains special chemicals that trigger ovulation in the female, causing the release of a mature egg from one of her ovaries. The egg descends from an ovary into a uterus and is fertilised. Once fertilised, the egg is encased in a fragile shell just a few microns thick and attaches itself to a rudimentary choriovitelline placenta, where the young koala grows. The koala female's pregnancy is very short, lasting just 33-35 days. She gives birth to a single baby koala called a 'joey'. On rare occasions, she may give birth to twins.
Before the birth of a young koala, the lining of the mother's pouch begins secreting an antimicrobial liquid to sterilise it. Her body also starts producing milk rich in immune cells and antibodies to protect her infant. She then takes up a "birthing position" and licks the opening of her birth canal to stimulate the birth.
At birth, the baby koala weighs about 0.5gms (0.02oz) and looks like a pinkish jelly bean. It is about the size of a human thumbnail and is blind, deaf, hairless, has tiny stumpy forelimbs and hardly a trace of its hind legs. However, even though it is still underdeveloped, the newborn has an excellent sense of direction and smell. Using its tiny forelimbs in a swimming motion, the joey instinctively crawls to its mother's pouch. There, it attaches itself to one of its mother's teats for nourishment and continues its growth.
The koala's pouch, located in the centre of the mother's abdomen, acts as an external womb for the tiny embryonic baby. This unique feature provides a safe and warm environment for the growing baby koala. The pouch is nearly furless and has two nipples. It is lined with muscles and ligaments that stretch to accommodate the growing joey inside. The pouch's forward-facing opening has strong sphincter muscles that can be closed to prevent the young baby from falling out.
Once inside the pouch, the young joey will stay hidden for about six months, protected and sheltered from the outside world. Then, as the joey gains confidence, it will tentatively pop its head out of the pouch and observe the world around it before eventually venturing out. The young joey will then hitch a ride on its mother's back for 6-12 months before becoming fully independent.
Interestingly a female koala isn't born with a pouch; it only develops as she approaches sexual maturity. Male koalas don't have a pouch. But just for the record male Tasmanian Tigers did.